We have further elucidated factors that prevent nosocomial pneumonia in patients intubated, and on mechanical ventilation (MV), using the sheep model. 1. CASS (continuous aspiration of subglottic secretions), now widely practiced clinically, was shown to be of no benefit, and was associated with significant tracheal mucosal injury. 2. A brief 24 h of MV, with head positioned as now clinically practiced, followed by extubation was associated (and in the absence of any adverse clinical findings) with high probability of significant bacterial colonization of the lungs, bursts of bacteremia, and high incidence of nosocomial pneumonia during the subsequent 2 days. 3. When sheep were intubated with tracheal tubes coated with silver sulfadiazine and chlorhexidine, and mechanically ventilated for 24 h, there was no bacterial growth in the tracheal tube, in the water trap, or in the inspiratory and expiratory lines of the mechanical ventilator. This preliminary finding is the first report of absence of bacterial growth in the ventilator circuit after 24 h of MV. 4. Combined with our earlier findings showing absence of bacterial growth in the trachea, and in the lungs, when the trachea and the tracheal tube were oriented below horizontal, our findings suggest preventive measures during mechanical ventilation that can be of importance in determining the presence/absence of bacterial colonization of the ventilator system, and of the upper and lower respiratory tracts.